They like to be in full sun. So with the summers of late, it maybe that it hasn't had the right conditions. Not knowing your location, I have no idea whether you have had good summers lately or not, but the last 3 yrs here in West Wales we haven't had it so warm, the hot summers are a thing to look back on, not sit and relax in at this time. Maybe it might improve. and if we do get any heat, my Pampas may flourish once again.

YOu don't say how long you've had the plant, but it wouldn't normally produce its plumes (flowers) until late September or October.